1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, which forms a document image on a sheet of paper by use of a developing unit containing toner.
2. Description of the Related Art
In an image forming apparatus (e.g., a copying machine), the surface potential of a photosensitive drum is controlled (i.e., the charge amount is varied by means of a charging grid), but this is done intentionally for the express purpose of forming an image of desirable quality or for forming an image of the best quality in accordance with the environmental conditions, such as the temperature and moisture. In general, the surface potential of the photosensitive drum is not controlled in accordance with a variation in toner density.
A developing unit is regarded as being in a toner empty state when the toner density of the developing agent contained in the developing unit has decreased to a predetermined value. Therefore, during the period between the time when the toner cartridge becomes empty and the time when the copying operation stops due to the toner empty state, the toner density in the developing unit is inevitably lower than the normal toner density. Since, in this state, it is likely that carriers will be attracted or attached to the photosensitive drum, a defective image may be formed.
In the conventional art, even when an output of the toner density sensor is greater than a reference value (which is a center value [4 V] in the case of FIG. 1 and corresponds to the time when the toner cartridge becomes empty), and the toner density lowers, the surface potential of the photosensitive drum remains at a center value (-700 V), as shown in FIG. 1. Therefore, during the period between the time when the toner cartridge becomes empty and the time when the copying operation stops due to the toner empty state, the number of carriers attracted to the photosensitive drum gradually increases, as shown in FIG. 2. In other words, the number of white dots produced in an all-black image increases, thus adversely affecting the quality of the image.
It is known in the art that carriers are attracted to the photosensitive drum when the toner density is low and the surface potential of the photosensitive drum is high, as shown in FIG. 3.
Therefore, the image quality is degraded due to the carriers attracted to the photosensitive drum during the period between the time when the toner cartridge becomes empty and the time when the copying operation stops due to the toner empty state.